Yes the caps pry off, they can be difficult some times. Under the cap is another piece of plastic that will need to be removed with a wrench/pliers. sometimes it's just some a piece of debri, plastic or copper shaving that is causeing the leak and after cleaning these pieces out and reinstalling them they work fine.

If they still leak after cleaning or if you just want to replace the parts you'll need to go to the plumbing supply house and get the vacuum breaker repair kit #35301 for a Model 17 wooodford hydrant. it will include the neccessary parts plus a replacement cap incase you break yours.